Karen Nadler was a main character in the first, second and third season. Karen was a scout for the 2nd Massachusetts Militia Regiment. She was romantically involved with Hal Mason in Season 1. Captured and harnessed by Skitters, she turned her back on humanity and began willingly serving the Espheni. Becoming a agent for the Overlord overseeing the eastern United States, she helped him hunt down a rebellion among the Skitters which led her to return to the 2nd Mass. Her efforts were in vain however, and the 2nd Mass destroyed a Espheni canon and killed her master. She served as the main antagonist of Season 3.

Karen was appointed by her masters to take his place, overseeing all military operations in eastern North America, including destroying the humans, the rebel Skitters, and the Overlords' interplanetary enemy, the Volm.

Karen was eventually killed by Maggie after the destruction of the Mothership in Boston, being shot multiple times after she tries to wave a white flag and work with the 2nd Mass.

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The day of the attack, she ditched school to climb a tower and see the spaceship. Had she stayed in school, she would have died with the rest of the students at Winthrop. She misses her family and joined the 2nd Massachusetts Militia Regiment to take revenge on those responsible for their death.

The night before the retreat from Boston, Karen is talking with her fellow scout and boyfriend, Hal. As they are discussing different motorcycles that they use for scouting, Lourdes, a civilian, approaches Hal. Karen glares at her warily, and after she walks away, Karen mocks Lourdes, causing Hal to grin.

When they arrive at the Sundash in Belmont, they find it mostly picked cleaned. Karen eavesdropped on the conversation between Tom and Weaver and the conversation about Tom putting together a squad to go back for supplies. She shouts out that he needs two scouts, implying she and Hal should go. Tom relents and tells them to find Dai, Click, Anthony, and one other to come with them.

After the rest of the 2nd Mass begin to move out, Karen, along with Jimmy a store in West Newton and a food distribution center in Watertown, Massachusetts, both places where there might be food. However, considering how close they were to the city center, they were most likely traps. He also points out that the main Skitter reinforcements would either come down River Road or Blanchard Road and they needed to know which one to avoid Skitters coming up behind them. He tasks Hal and Karen to scout the two roads to find out which one they're coming down, while the rest of them wait at the train station.

After Tom Mason's team is ambushed and captured in the armory by Pope's Gang, Karen is held as a hostage. John Pope - who dubs Karen "Sexy Freedom Fighter Girl" - demands a .50 cal machine gun in exchange for the hostages' safe release. After Billy Pope implies that he intends to rape Karen - as he ostensibly did to Margaret - Margaret betrays Pope's Gang and releases Karen.

While attempting to rescue Ben Mason, Hal and Karen are captured by the Skitters and Mechs that are guarding the harnessed children. Tom Mason and his squad are forced to abandon them. After being stunned and unconscious for many hours, Hal slowly wakes up only to witness Karen being dragged away by harnessed children.

It is revealed that she has been harnessed and was being used by the Overlords to communicate with one of their agents, Sonya Rankin. When Hal sees her he wants to save her, but stopped by his father who says that it isn't the time.

Karen would later be used by the Overlords to convince Tom Mason to willingly give himself up in exchange for Ben's life. After the attack on Boston Karen appears in the middle of the road, stopping Weaver and Tom, and convinces Tom is take board on the alien mothership, saying if he didn't they could call back Ben.

While Weaver recovered from the serious infection caused by the recent attack, Tom Mason decides to send out crews looking for diesel fuel. Margaret and Hal stumble upon a group of dead and naked unharnessed kids buried in a pile of leaves, with Karen being the only one alive.

While recuperating throughout the episode she showed signs of increased strength, hearing and inhuman agility and speed. She also began to carefully manipulate those around her starting first with Hal Mason and then with Ben Mason. Hal catching Ben and Karen in a close moment, Ben runs off. Hal tells Maggie he thinks Ben was trying to recruit her, as their spikes were glowing.

Maggie attempts to kill Karen, in which Karen effortlessly disarms and wounds Maggie by throwing her against a wall, before Ben escapes with her to the roof. As they figure where to run, Hal stops them at gunpoint. Ben says sorry, before knocking Hal out. They then jump off and run into the woods. Pope, who returns earlier in the episode (and is passed out with a concussion), comes to, and talks about having been interrogated by Karen. He describes Karen as frightening and capable of reaching into your mind and forcing you to give her answers. It is revealed that the Overlords know of the Skitter rebellion led by Red Eye and are trying to find it. They sent Karen back to the 2nd Mass because they knew that Ben had been recruited into the movement and they needed to find it.

Karen asks Ben about where they're going. Ben says he's trying to find Red Eye and the Skitter rebellion. Karen begins to ask Ben about the resistance, where they are, how he came in contact with them. When Ben answers these questions Karen springs her trap revealing that she's been looking for him. Karen is re-Harnessed by the Overlord (and tries to seduce Ben into doing the same), Tom and the Berserkers arrive to rescue Ben, managing to capture the Overlord when the Mechs around them cease fire because they don't want to risk hitting it. Karen then had the Mechs surround the hospital, ordering the 2nd Mass to release the Overlord. Karen attempts to negotiate a cease fire by raising a white flag and asking Tom to come out and talk to her. Tom and Hal come out to talk to her, Karen tells the two that they need to give up her master and the alien forces will let them go. Tom refuses stating that they'll just kill them all afterwards. Hal is unresponsive and dismissive of Karen. The negotiation reaches an impasse as Tom and Hal ask to let them go anyway, but Karen believes they will kill the Overlord as soon they get away, Karen doesn't believe they can trust the 2nd Mass, Tom and Hal walk back towards the 2nd Mass. Later Karen, brings out Boon, who was captured during a scouting mission, she orders him to walk back to the 2nd Mass, only to have a Mech behind her slowly execute him. Karen is escorted by Hal to the Overlord and reveals that she wishes things could have been different. Hal is dismissive towards her, shoving her through the hall at gunpoint. Karen states that Hal could never understand the meaning of what the Overlords are trying to accomplish. Hal rejects that saying it's biggest load of crap he had ever heard. Karen confronts Hal stating she organized the killing of the kids and the placing of their body’s so Hal would find her. She states that if someone knows what events are going to occur, they can alter them. Hal responds by saying they missed a few things, she says there are always variables. But that the end result is the same which is why they are giving her master up.

Karen is shown to be noticeably upset when she sees the Overlord wounded, she tries to attack Tom but is held back by Ben. Tom says to let them all go and quickly or the Overlord will die. Karen is then shown several blocks of explosives that will be detonated if the 2nd Mass is not released, killing her and the Overlord, she then allows them to leave.

Karen and her Overlord master are supervising the construction of a new weapon on the outskirts of Charleston, South Carolina when the 2nd Mass attacks the complex to destroy the weapon. As they are setting up the bombs, Skitters, led by Karen, attack them, killing Dai and capturing the rest. She has them tied up so that she can interrogate them by torturing them with an electrostaff while her master watched. She also tortures them psychologically by kissing Hal to enrage Maggie and threatening Anne to throw off Tom. During this time, she discovers that Anne is pregnant and threatens to electrocute both Anne and Tom's unborn child. Tom breaks and tells her that he'll talk.

At that moment however, the rebel Skitters led by Red-Eye attack the complex where they release the human prisoners and kill the loyal Skitters in order to obtain their true objective: the death of Karen's master who commands all military operations on the Eastern seaboard of the United States. Red-Eye engages the Overlord but is killed after a staunch fight, while Tom seizes the electrostaff and kills the Overlord by smashing his brains in. Realizing the battle is lost, Karen yells to Tom that it isn't over and climbs up the wall to escape.

It is later revealed that when Karen kissed Hal, she implanted an eye worm, similar to the one used by Red Eye inside him, apparently leaving him under the aliens' control.

The resistance were first informed of Karen's 'promotion' by the rebel Skitters acting as spies in Espheni ranks. The resistance, as well as having formed an alliance with the Skitters, had formed an alliance with the Volm, the Espheni's intergalactic enemies, who provided them with the weaponry that they needed to win the war against the Overlords. The Skitters warned Tom Mason- now President of Charleston and the New United States- that Karen would prove even more dangerous than her predecessor, suggesting she had already re-coordinated the Espheni's military deployments.

For seven months, the resistance hit the Espheni in a guerrilla type of warfare, forcing Karen to ressort to espionage to get an advantage over them. Having already placed a probe in Hal Mason, Karen neverthless soon discovered that Hal was resisting the effects of the probe. While she groomed him at night in what he thought were dreams, she also somehow managed to place a number of probes in Lourdes Delgado. She soon begun acting as Karen's primary spy in Charleston.

Thanks to Lourdes, Karen set a trap for the resistance using a number of deharnessed children as bait, as they worked in a mine. A force led by Weaver and Tom Mason attacked the location to save the harnessed children and fell into the trap because Karen had chosen to deploy her Mega Mechs against them. Only the Volm's advanced weaponry managed to save them.

Later, Karen succesfully managed to corrupt Hal and used him to kidnap Anne Glass and her daughter Alexis so that she could lead Tom into a trap. When this plan failed, Hal kidnapped Tom but failed to leave Charleston before he was discovered. Karen's plan to find out about the Volm weapon had failed and the rebel Skitters were able to extract the probe from Hal.

Later, Karen's forces captured Tom and she subjected him to torture by, via an Espheni device, showing him scenes of his old life which were mixed with the presence of his post-invasion acquaintances such as Anne. Tom pulled out of it, preventing Karen from discovering where the Volm weapon would be deployed; angry that she had failed, she led him into another room where the bodies of Anne and Alexis- apparently dead- were being kept, causing Tom to fly into a tearful rage. She also made Tom witness the activation of the planetary defense grid which would prevent the Volm forces from landing and also, via radiation, kill off the remaining humans. However, Tom manages to escape from the Mother ship.

After the destruction of the Boston Mothership and the landing of the Volm troops, Karen tries to parley with the resistance in the hopes of turning them against the Volm and even trying to form an anti-Volm alliance with the humans; it was revealed at the end of the episode that she actually kept Anne and Alexis alive, and brought them back to Tom as a gesture of good faith. However, Tom vengefully shot her in the stomach. And after they destroyed her escort of Skitters and Mechs, Karen tried to have a conversation with Hal, seeming to return to her former self. Hal seemed to give in but Maggie coldly shot her twice, killing her and ending the threat she had posed to the resistance since she was taken.

During one of Annes memories, she dreams about Karen when she puts her and Alexis into the cocoon. Karen tells Anne that Alexis will become just like her, but more powerful. Later, when an adult Alexis shows up in Anne's subconscious, she shows her that she chose Anne over Karen, as Karen wanted to take her away to The Monk and end the war with her powers.

In an effort to turn the resistance against their enemy, the Volm, Karen brought a small troop of Espheni forces and waved a white flag to try and negotiate with Tom Mason. Karen tells Tom he has a present for him, in and effort to persuade him, however Tom has enough of Karen and shoots her in the stomach. While Karen is bleeding out, Hal kneels down to hold her hand before she passes, but Maggie shoots her twice in the stomach, killing her.[1]